1. Field of the Invention
The invention relates to microemulsion compositions having improved flammability characteristics, and especially to fire resistant microemulsion fuel compositions--i.e., compositions which are not ignited by an open flame or pools of which are self-extinguishing following ignition. These compositions are of special importance in safety applications, for example, for use underground in mines, in combat vehicles, on aircraft carriers, and the like. In accordance with the present invention there are provided microemulsion compositions which have higher flash point characteristics thus providing improved flammability. In a special application microemulsion fuel compositions e.g., diesel formulations, are provided which are fire resistant.
2. Description of the Prior Art
Microemulsions are clear, stable, two-phase dispersions which form on simple stirring under appropriate conditions. Water in oil microemulsions (w/o) are comprised of a continuous non-polar hydrocarbon phase and a discontinuous aqueous phase. Because of the small droplet size of the discontinuous phase (2 to 200 nanometers) these microemulsions appear to be clear, one-phase systems.
The effects of water addition on diesel engine performance is reviewed in "Water and Alcohol Use in Automotive Diesel Engines", DOE/CS/50286-4, published September 1985 by J. J. Donnelly, Jr. and H. M. White. The techniques for introducing water into the engines covered by this review included (macro) emulsification, blending, fumigation, and dual-injection. The introduction of water was found to reduce emissions of smoke and particulates 20-60% while moderately reducing or increasing emissions of hydrocarbons and carbon monoxide. The addition of water also reduced levels of NOx 10-50%. This held true for all methods for introducing the water, and is attributed to a lowering of combustion temperatures (due to lower specific heating values and the heat absorbed to vaporize the water or alcohol droplets), and to a "microexplosion" phenomenon (the dispersed droplets vaporize explosively, more effectively atomizing the hydrocarbon fuel during combustion).
Water is most advantageously introduced into combustion engines dispersed in the hydrocarbon fuel as a microemulsion. Since microemulsions are clear, stable, and pre-blended (prior to being stored in the fuel tanks), there is no need for additional equipment on the vehicle (as would be required for the other methods) such as additional fuel metering systems (dual-injection), agitators inside the fuel tanks (to prevent separation of macroemulsion fuels), injection or fumigation devices, etc. At the same time, the water is still introduced into the engine in the desired physical form, i.e., as microscopically fine liquid droplets (albeit dispersed as micelles in the hydrocarbon), preserving the ability to vaporize in the desired "microexplosion" manner.
An excellent general treatment of the subject of microemulsion fuel compositions is "Microemulsion Fuels: Development and Use" ORNL TM-9603, published March 1985 by A. L. Compere et al. Again, the presence of water (in microemulsions) led to large reductions in smoke and particulates, with slight increases in hydrocarbons and CO emissions. Depending on the type of engine used and operating conditions, NO.sub.x emissions were moderately decreased or increased.
Research sponsored by the U.S. Army Fuels and Lubricants Research Laboratory investigated the effect of water-in-fuel microemulsions on the fire-safeness of combat fuels. Several reports by W. D. Weatherford, Jr. and coworkers (AFLRL reports Nos. 111, 130 and 145) document the effectiveness of microemulsion diesel fuels containing 1-10% water in reducing the flammability--fuel pools were either self-extinguishing following ignition, or could not be ignited by an open flame. The Army formulations were prepared with deionized water and surfactants without the addition of alcohols as cosurfactants. If low levels (200-500 ppm) of dissolved salts were present in the water, stable microemulsions could be formulated only by substantially increasing the percentage of surfactants, or by increasing the aromatic hydrocarbon content of the fuel. Even then, the amount of water that could be incorporated into the fuels were reduced when salts were present.
Various patents have issued which relate to microemulsion fuel compositions and which specifically relate to compositions comprised of hydrocarbon fuel, water, various alcohols, and surfactants. U.S. Pat. No. 4,046,519 for example, teaches a microemulsion fuel comprised of gasoline, methanol, water and a surfactant blend having a hydrophilic-lipophilic balance value of 3 to about 4.5. U.S. Pat. No. 4,083,698 describes fuel compositions which are water-in-oil emulsions and which comprise a hydrocarbon fuel such as gasoline or diesel fuel, water, a water-soluble alcohol such as methanol, ethanol or isopropanol, and certain combinations of surface-active agents. U.S. Pat. No. 4,451,265 describes microemulsion fuel compositions prepared from diesel fuel, water, lower water-miscible alcohols, and a surfactant system comprising N,N-dimethyl ethanolamine and a long-chain fatty acid substance. U.S. Pat. No. 4,451,267 teaches microemulsions prepared from vegetable oil, a C1-C3 alcohol, water and a lower trialkyl amine surfactant. This patent teaches the optional addition of 1-butanol as a cosurfactant for the purpose of lowering both the viscosity and the solidification temperature of the microemulsion.
U.S. Pat. Nos. 3,527,581 and 3,346,494 describe microemulsions of various fuels and lubricants using a particular combination of emulsifiers, one component of which is phenol containing a higher alkyl group substituent.
In co-pending application Ser. No. 825,841 filed Feb. 4, 1986, microemulsion compositions of improved stability are provided through use of a cosurfactant system containing tertiary butyl alcohol as one component. Although the formulations described in said co-pending application demonstrate superior stability, and substantial reduction in particulate and smoke emissions, they have low flash points and thus have flammability characteristics no different from the starting hydrocarbon fuels.